The present invention relates generally to automated process planning for quality control inspection.
A related patent application by Steven R. LeClair et al titled "INDUCTIVE-DEDUCTIVE PROCESS DESIGN FOR MACHINED PARTS", Ser. No. 08/159,968, filed Nov. 30, 1993, discloses a feature-based technique which, along with the present invention, are part of a Rapid Design System.
The task of the Quality Assurance (QA) engineer is to determine if the geometries of the product are within the specified tolerances created by the design engineer. The results should determine whether or not the product will perform its desired functions correctly. Today's technologies have provided a diverse range of automated inspection systems for QA. The most popular have been vision systems and coordinate measurement machines (CMM), which are beginning to be used extensively in industry for automated industrial inspection of machined parts. Advanced graphical programming tools have also simplified some aspects of the automated inspection process. Computer-aided design, process planners, path planners, and simulators provide assistance to the inspector.
The CMM has become very popular due to new technologies increasing its speed and accuracy, allowing more than 60 measurements per minute with accuracies to 0.00001 inch. Therefore, the new challenges surrounding CMMs are not seen in the act of data retrieval, but rather the planning before and after data acquisition and retrieval--where to take the inspection measurements, how to sequence the inspections for efficiency, and how to evaluate them once they are taken.
It is quite time-consuming for an inspector to understand a drawing or blue-print and determine how to inspect the part. The interaction of geometry/function/tolerance is not apparent without careful study. Recently, there is a new government tolerancing language standard--Geometric Design and Tolerance (GD&T)--which is also emerging as the industry standard. This will help to standardize the communication between the designer and the inspector, but even an experienced inspector must interpret the designer's intent for the workpiece functionality, which is what inspection techniques are based upon.
Once the relationship between part functionality and tolerance callouts are established by the inspector, the process plan is created. The functionality of the part is first used to specify the setup orientations, which define the fixturing, if needed. Throughout the planning process, some tasks are inherently more difficult to the inspector than others. The tasks mentioned here--creating setups, placing fixtures, and determining coordinate frames--are not terribly cumbersome for the experienced Inspector. Other tasks to follow, like calculating the measurement points and via points (specified to a degree of accuracy, usually in the thousanths of an inch), will prove much more work intensive and critical to the inspector.
Often there are several coordinate frames within a setup, meaning that three dimensional coordinate transformations must be performed to calculate the inspection coordinates from the designer coordinate reference frame. From our study of inspectors, this is done with paper and pencil using a handheld calculator--even with a Masters degree in mathematics, this task is difficult and takes a large amount of time!
The measurement points must then be sequenced. An inspector will use various criteria for ordering the points. These vary from shortest path distance, to tolerance/evaluation organization. From our studies, we noticed that often the points are sequenced so that variant process planning--creating a new process plan from editing the process plan of a similar part--is most easily performed. This entails grouping together points from the same tolerance evaluation for the ease of cutting and pasting for a similar part.
It is imperative that the sequenced path is checked for collisions. As is sure to be the case, the path connecting the sequenced measurement points will intersect with the workpiece or fixtures. To correct this, via points are created to avoid the obstacle. This task is very time consuming for the inspector. The chance of human error is high since the via point coordinate must be found from the design specifications and then translated into the coordinate reference frame that contains it. It is also very important that this be cone accurately, since an error in calculation can cause a collision, damaging a very expensive probe. This process has a high risk assessment since there is a rather high probability of error with a high cost from an error.
Finally, the process plan is translated to the instruction language of the CMM performing the measurement and evaluation. The inspector usually types this into his word processor from the process plan sketched out on several handwritten notes. Not only are these inspection languages cryptic and hard to remember, but the keyboard entering of the code is extremely important. One small typographical error or misplaced minus sign could cause a large undesirable result: an improperly evaluated tolerance resulting in erroneous acceptance or rejection of tolerance; a measurement of a non-existing point halting the process plan; or worse, a probe crash into the workpiece, a fixture, or the CMM table top. This process also has a high risk assessment and must be re-checked several times for accuracy.
Some CMM vendors have created computerized aides for the inspector. They simply consist of macros where the inspector fills in blanks with his own coordinate point calculations. This helps the inspector tackle the problem of remembering some of the cryptic CMM instructions, but does not help create the precise coordinate points that must be entered, and sequence both the setups and the measurement points within the setups. Research in automated inspection planning to date has discussed development of inspection planners for only turned parts and sculptured surfaces to the best of our knowledge.
The following United States patents are of interest.
Simoudis U.S. Pat. No. 5,208,768
Marmelstein U.S. Pat. No. 5,187,788
Newell et al U.S. Pat. No. 5,123,087
Carver et al U.S. Pat. No. 5,023,800
The patent to Simoudis discloses a discrepancy determination component that identifies a discrepancy between operation of an original design and a desired operation. The patent to Marmelstein discloses an automatic code generation tool which allows the programmer to quickly create a graphical representation of the programmer's initial program design. The patent to Newell et al discloses a computer aided drafting system and methods for automatically locating geometric points. The patent to Carver et al discloses a 3-dimensional computer system having a graphics definition of an article located in a system memory.
It can be quite time-consuming for an inspector to understand a drawing and determine how to inspect a part. Programming CMMs is also time-consuming, tedious, and highly susceptible to human error. This overhead is particularly critical when it is spread over a small number of parts.